Pump impeller assembly

ABSTRACT

A pump impeller assemly for reducing cavitation while maintaining a high pump performance, which assembly comprises a main impeller of centrifugal type and a front impeller of diagonal-flow type with a few blades. The two impellers are disposed with suitable relationship among radial and peripheral spacings between the impellers.

Unite States Kida et a1.

atent [191 1 Nov. 11, 1975 1 PUMP IMPELLER ASSEMBLY [75] Inventors: Kazuo Kida; Naokazu Kubota, both of Mishima, Japan [73] Assignee: Dengyosha Machine Works, Ltd.,

Tokyo, Japan [22] Filed: Feb. 1, 1974 Appl. No.: 438,788

Related U.S. Application Data [63] Continuation-in-part of Ser. No. 313,933. Dec. 11.

1972. abandoned.

[52] U.S. C1 416/183; 416/201 [51] Int. Cl. F04D l/06 [58] Field of Search 416/175, 183, 200, 201,

416/203; 415/195, DIG. 1

[56] References Cited UNITED STATES PATENTS 2.753.808 7/1956 Kiuge 416/183 3,588.280 6/1971 Yedidiah 416/201 X FOREIGN PATENTS OR APPLICATIONS 76.077 6/1947 Czechoslovakia t. 416/183 573.559 4/1933 Germany 416/183 Primary Eraminer-Everette A. Powell, Jr. Atmrnev, Agent, or Firm-Silverman & Cass, Ltd.

[ 5 7] ABSTRACT A pump impeller assemly for reducing cavitation while maintaining a high pump performance, which assembly comprises a main impeller of centrifugal type and a front impeller of diagonal-flow type with a few blades. The two impellers are disposed with suitable relationship among radial and peripheral spacings between the impellers.

2 Claims, 6 Drawing Figures US. Patent Nbv.11, 1975 SheetIc f3 3,918,841

F/GLZ PRIOR ART PUMP IMPELLER ASSEMBLY CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of our copending US. application Ser. No. 313,933 now abandoned, filed Dec. 11, 1972, and assigned to the same assignee of the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a pump impeller assembly, and more particularly to a pump impeller assembly suitable for the use in a high-speed centrifugal pump.

2. Description of the Prior Art In raising the rotating speed of a pump, it is necessary to suppress cavitation, because cavitation tends to diminish the pump performance.

With conventional centrifugal pumps, three approaches have been known by which suction chacteristics of the pump can be improved so as to suppress cavitation; namely, (1) by widening the flow passage at the blade inlet of an impeller, (2) by reducing the number of blades, and (3) by increasing the blade inlet angle. There is a limit in actually applying such known approaches. It has been difficult, with conventional approaches, to reduce the cavitation performance without impairing the pump performance.

Due to such limitation in the conventional approaches which rely on the imporvement of pump suction performance by modifying the configuration of the impeller per se, it has been very difficult to increase suction specific speed S in excess of 1,500. The suction specific speed S is indicative of the extent of cavitation and defined by the following equation:

5 n ilz 314 where n: pump speed, in revolutions per minute,

Q: flow rate, in m lmin,

H net positive suction head, in m.

To greatly improve the suction performance of a pump, it has been proposed and practiced to increase the suction pressure of a centrifugal impeller by disposing an inducer (an impeller of axial-flow type) in front of the centrifugal impeller. The use of such inducer, however, has a shortcoming in that the length of the pump shaft tends to become too long. Especially, in double suction pumps, the pump shaft tends to be too long in the axial direction. Furthermore, the fluid flow is disturbed at the space between the inducer outlet and the centrifugal impeller inlet, whereby the pump performance is impaired. The use of the inducers, of course, means an additional cost of the pump.

Therefore, an object of the present invention is to mitigate the aforesaid difficulties in the conventional pump by providing an improved pump impeller assembly.

SUMMARY OF THE INVENTION According to the present invention, there is provided a pump impeller assembly, which is of the so-called tandem impeller type, comprising an impeller of centrifugal type (to be referred to as a main impeller) and another impeller of diagonal-flow type (to be referred to as a front-impeller) disposed in the proximity of the blade inlet end of the main impeller. The number Z oF blades in the front-impeller to be used in the present invention is two or three, and the number of blades in the main impeller may be an integral multiple of the corresponding number Z in the front-impeller.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention, reference is made to the accompanying drawing, in which:

FIG. 1 is a schematic sectional view of a conventional pump having a conventional inducer of axial-flow type;

FIG. 2 is a schematic sectional view of a pump incorporating a pump impeller assembly according to the present invention;

FIG. 3 is a diagrammatic illustration of relative disposition of a main impeller and a front-impeller in the impeller assembly of FIG. 2;

FIG. 4 is a graph showing the performance characteristics of a pump using an impeller assembly of the invention;

FIG. 5 is a graph showing the cavitation characteristics of a pump using an impeller assembly of the invention; and

FIG. 6 is a graph showing the variation of suction specific speeds at head break down, for different relative dispositions between the front-impeller and the main impeller.

Like parts are designated by like numerals and symbols throughout the different figures of the drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention will now be described in detail, by referring to the accompanying drawing. In a conventional pump, as shown in FIG. 1, a main impeller 1 has an inducer 2 of axial-flow type which is disposed upstream of the main impeller. Both the main impeller 1 and the inducer 2 are mounted on a pump shaft 4. Since the inducer 2 is of axialflow type, the overall length of the pump of FIG. 1, taken in the longitudinal direction of the shaft 4, is very long. The pump includes a spiral casing 5 and a bearing 6 which is located adjacent to the spiral casing. A suction casing 7 extends upstream of the spiral casing 5, so as to enclose the main impeller 1 and the inducer 2.

With the present invention, a front-impeller 3 of diagonal-flow type is disposed adjacent to the blade inlet of a main impeller 1, as shown in FIG. 2. As is apparent from comparison of FIGS. 1 and 2, the use of the frontimpeller 3 of diagonal-flow type results in a very short overall length of the pump.

Referring to FIGS. 2 and 3, the main impeller 1 and the front-impeller 3 of the invention are mounted on a common pump shaft 4, with a spacing s taken along the direction of flow line in the meridian plane, and the blades of the main impeller 1 and the front-impeller 3 are displaced by a distance it in the circumferential direction of the impellers. In FIG. 3, the chord length of the front-impeller blades in the meridian plane is represented by l, and the pitch of front-impeller blades at the outlet end is represented by c. In FIG. 2, the diameter of each eye-shaped portion at blade inlet end of the main impeller 1 is represented by symbol a, and the pitch of the blades of the main impeller 1 at its inlet end is represented by symbol t.

In addition to the reduction of the overall length of the pump, the front-imp eller 3 of the pump impeller assembly of the present invention acts to greatly improve cavitation performance of the pump. The combination of the main impeller l with the front-impeller 3 results in a composite impeller assembly having partially elon- 3 gated blades, so that the pump performance is improved.

FIG. 4 illustrates performance characteristics of a pump incorporating an impeller assembly of the invention including a main impeller with six blades and a front-impeller with two blades, which are assembled under the conditions of s/a 0.2, 11/! 0.33, and I 1.16. In the figure, the dotted lines represent similar performance characteristics obtained by using the main impeller alone. It is apparent from the figure that, with the pump impeller assembly of the present invention, the pump performance is improved as compared with that of the main impeller alone, except for unduly large flow rates.

FIG. illustrates an example of cavitation performance of a pump incorporating an impeller assembly including a main impeller with six blades and a frontimpeller with two blades, which are assembled under the conditions of s/a 0.2, h/t 0.33, and l/c 1.16. The figure shows the variations of total pump head H, in meters, and efficiency v, in percent, for different values of net positive suction head H in meters, under the conditions of flow rate Q at 4.8 m /min. For comparison, similar characteristics are shown in the figure for the case of using the main impeller alone. As the net positive suction head H decreases, the total pump head H suddenly drops due to cavitation. With the main impeller alone, such sudden drop of the total pump head H occurs when the suction specific speed S is 2,180. However, when the impeller assembly of the present invention is used, the head breakdown occurs at the suction specific speed S of 4,150. Similar comparisons may be made on the points at which the total pump head H begins to become smaller due to start of cavitation; namely, With the main impeller alone, the head reduction begins at the suction specific speed S of 1,250, while with the impeller assembly of the invention, it occurs at the suction specific speed S of 3,610, so that the pump performance in this respect is improved by a factor of about three times. Such improvement of the pump performance is made possible because cavities generated in the front-impeller enter only those inter-blade passages of the main impeller which are contiguous to suction pressure sides of the blades of the front-impeller, but do not enter into those inter-blade passages of the main impeller which are contiguous to the pressure sides of the blades of the front-impeller.

FIG. 6 shows the variation of suction specific speed at breakdown of total pump head for different relative positions between the front-impeller and the main impeller of a pump impeller assembly of the invention. In the figure, the ordinate represents different positions of the front-impeller in the rotating direction of the impeller, in terms of the ratio h/z, while the spacing between the main impeller and the front-impeller in the flow direction, in terms of the ratio s/a is used as a parameter. As apparent from FIG. 6, satisfactory result can be achieved for the ranges of 0.2 s (ll/I) s 0.6 and 0.1

(s/a) s 0.6. Furthermore, it was confirmed by tests that when the Ne ratio is smaller than 0.7 or greater than 1.25, the fluid flow becomes unstable and the object of the invention cannot be achieved.

As described in the foregoing disclosure, with the pump impeller assembly of tandem impeller according to the present invention, the overall length of a pump can be reduced, as compared with that having an inducer of axial-flow type, and the risk of cavitation can be diminished for a wide operating range, without diminishing pump performance.

We claim:

1. A pump impeller assembly comprising, a centrifugal type main impeller, the blade inlet end portion of the main impeller being of generally eye-shaped configuration, a onestage front-impeller with at least two blades, said front impeller being disposed just in front of the main impeller relative to the flow direction of fluid through the assembly with the following relations:

01 S (x/a) 5 0.6 0.7 5 (Ur) S 1.25 0.2 s (h/t) S 0.6

where a is the diameter of the eye-shaped blade inlet end portion of the main impeller, s is the spacing from the blade inlet end of the main impeller to the blade outlet end of the front-impeller taken in the direction of line of flow through the assembly, I is the blade pitch of the main impeller at the blade inlet end thereof, h is the relative spacing between the inlet end of the blade of the main impeller and the outlet end of the blade of the front-impeller taken in the peripheral direction of the impellers, l is the front-impeller blade chord length in the meridian plane, and c is the pitch of the frontimpeller blades at their outlet.

2. A pump impeller assembly as claimed in claim 1, wherein the number of blades in the main impeller is an integral multiple of the number of blades in the front- 

1. A pump impeller assembly comprising, a centrifugal type main impeller, the blade inlet end portion of the main impeller being of generally eye-shaped configuration, a onestage front-impeller with at least two blades, said front impeller being disposed just in front of the main impeller relative to the flow direction of fluid through the assembly with the following relations: 0.1 < OR = (s/a) < OR = 0.6 0.7 < OR = (l/c) < OR = 1.25 0.2 < OR = (h/t) < OR = 0.6 where a is the diameter of the eye-shaped blade inlet end portion of the main impeller, s is the spacing from the blade inlet end of the maIn impeller to the blade outlet end of the front-impeller taken in the direction of line of flow through the assembly, t is the blade pitch of the main impeller at the blade inlet end thereof, h is the relative spacing between the inlet end of the blade of the main impeller and the outlet end of the blade of the front-impeller taken in the peripheral direction of the impellers, l is the front-impeller blade chord length in the meridian plane, and c is the pitch of the front-impeller blades at their outlet.
 2. A pump impeller assembly as claimed in claim 1, wherein the number of blades in the main impeller is an integral multiple of the number of blades in the front-impeller. 